i don't use mac, and most regulars here do not either. i can give some input based on the hardware. i think the 21" screen size is just too small for working with creative software. there are other limitations to the 21" models i will point out in the hardware sections below. if these machines will be split up and only have to handle some of the software, like only photoshop/illustrator, then it might make sense to have multiple configs. i take it the gtx 680's are in the small tree server? if you are actually using the gtx 680 with the mac pro's via e-gpu, then it may be possible to use those to with the imac's. if you weren't planning on doing this already, it would be good to order one or two machines with the configuration(s) you think will be best and test them out, before committing to ordering bulk. cpu: the entry level 21" imac with the 1.6ghz i5 should be skipped, its using a laptop i5 cpu and is much slower than the desktop i5's in the other models. photoshop and illustrator would probably be ok with a desktop i5. audition might also but would depend on how many and which vst/plugins are being used, and how many audio tracks. AE and premiere will be the most demanding and may benefit the most from the i7. i've heard the imac can overheat and thermal throttle the i7 during extended max cpu usage sessions like exporting/rendering. ram: you might already have an idea of how much memory the machines would use, based off the current mac pro's. if not, AE and premiere will likely need the most, between 16-32gb, while the rest of the software may be fine with 16gb. only the 27" holds up to 32gb (4x8gb), while the 21" imac's can only hold 16gb (2x8gb) max. i think memory upgrades are only simple with the 27" imac. third party apple compatible memory is also available, which is cheaper than apple branded memory. with the 27" imac, if you need to save costs you could start with 16gb (2x8gb) and add more later if needed, or start with the default 8gb (2x4gb) and add in 16gb (2x8gb) of third party memory for a total of 24gb. storage: it would be nice to use a flash drive instead of the 7200rpm hdd or fusion. how large will depend on what it needs to hold locally, like any cache or any project files etc. if everything is stored on the network, the small 256gb flash drive may be enough. if budget is tight and the cache and media files will be on the network, the fusion drive might be a good compromise over the 7200 rpm hdd. i think the drive is buried inside the imac's and are very difficult to upgrade later. usb 3 or thunderbolt external storage would be an easier upgrade. gpu: if the premiere or AE projects will be using alot of gpu fx or third party gpu plugins, the 27" imac with the R9 395 would offer more power to handle those. if using just a few gpu fx, the M380 or M390 would help cut costs. i think there are some plugins for photoshop that can also benefit from a good gpu, but i think those plugins are kinda rare. if planning on doing any 4k projects the extra vram on the R9 395X 4gb may help. the 21" imac only has intel graphics, which wouldn't be good for premiere or AE. an e-gpu via thunderbolt may allow for a gpu upgrade later if needed, or also allow for nvidia gpu with cuda if software requires.
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